1. Field of the Invention
The present invention relates to a process for producing a magnetic fluid, and more particularly to a process for producing a magnetic fluid having an improved saturation magnetization by stably dispersing fine particles of ferrites in a base oil of low vapor pressure at a high concentration.
2. Discription of the Prior Art
Fine particles of ferrites are produced by pulverization, coprecipitation or vapor deposition, and a coprecipitation process is preferably used from the viewpoints of purity, particle size control and productivity. However, the coprecipitation process is based on a precipitation reaction in an aqueous solution containing iron ions, and thus the fine magnetic particles are obtained in an aqueous suspension.
On the other hand, it is desirable that fine magnetic particles for a magnetic fluid are discretely dispersed in a liquid without any coagulation. In case of producing a magnetic particles by coprecipitation, it is necessary that a surfactant for preventing deposition and coagulation is adsorbed onto the surfaces of fine particles in a dispersion state without any drying step involving a risk of deposition and coagulation of fine particles themselves. To this end, a water-soluble surfactant is used.
In a magnetic fluid containing a water-soluble surfactant-adsorbed, fine magnetic particles in a dispersion state, the base oil for dispersion is restricted to solvents having a relatively high volatility such as kerosene and toluene. When a magnetic fluid is applied to magnetic fluid sealing or magnetic fluid polishing, evaporation of the base oil will deteriorate the function of the magnetic fluid itself.
A magnetic fluid is a dispersion of fine particles of ferrites in a base oil, dispersed usually with a dispersing agent such as a higher fatty acid salt or sorbitan ester. When fine particles of ferrites are to be simply dispersed in a base oil of low vapor pressure, any higher dispersibility cannot be obtained, and the resulting dispersion has no practical value.
Even if a good dispersibility could be obtained in the dispersion into such a base oil of low vapor pressure, the base oil of low vapor pressure has a dynamic viscosity as high as about 8 to about 50 Cst (40.degree. C.) in contrast to ordinary organic solvents and water having a dynamic viscosity of less than 1 Cst (40.degree. C.), and thus it takes a very long time to form a homogeneous suspension. Furthermore, all the fine particles of ferrites are not always formed into a stable suspension, and a considerable proportion of fine particles of ferrites is removed during purification such as centrifuge, resulting in very poor yield.